The present invention relates to flash units.
More particularly, the invention relates to a computer-controlled electronic flash unit.
Modern electronic flash units are broadly divided into those having a built-in computer and those which do not have such a computer. In the latter ones the flash intensity is always the same, since during each flash operation the total energy stored in the flash condenser is supplied to the flash tube.
Sometimes, however, it is desired to be able to vary the light intensity of the flash in dependence upon the distance of the object to be photographed, or upon the selected aperture setting. To make this possible computer-controlled electronic flash units have been developed. These have a light-responsive sensor which switches the flash tube off when light reflected from the object reaches a preselected threshold value.
Computer-controlled flash units are either of the series type or the parallel type. Units of the parallel type have a switch which is connected in parallel with the flash tube, whereas in the other type the switch is connected in series with the flash tube. In the parallel type flash units the flash capacitor is fully discharged during each flash operation; in the series type, however, this is not the case since the switch (which closes when the threshold value is reached) blocks further supply of electrical energy to the flash tube. Therefore, the series type units are energy saving since a greater number of flashes can be produced with them per set of batteries (or per charge of the accumulator) than with the parallel type units.
These flash units are self-contained, in that they each have all essential required components. The only external influence, namely the need to connect the units to a camera, has the sole function of supplying the flash unit with a trigger signal while the camera shutter is in open position. Such flash units might therefore be considered as quasi-autonomous units.
There are, however, also computer-controlled electronic flash units which are controlled by a sensor mounted in the camera, rather than in the flash unit itself. For example, German Allowed Application DT-OS No. 1,797,079 discloses a flash unit wherein a photoelectric cell is provided in the unit and another in the camera with which the unit is to be used. Each of these cells can be electrically connected with the flash-duration circuit of the unit to control this circuit and at the same time the respectively other cell will be automatically disconnected from this circuit.
However, it is desirable to be able to use also the conventional, currently marketed computer-controlled electronic flash units with the type of camera having a built-in flash control system. No solution to this problem has heretofore become known.